What is a single-storey extension?
A single-storey extension is a ground-floor addition, usually at the rear of a house, that enlarges a kitchen, living or dining space. Most are built with cavity walls on concrete foundations and finished with a flat roof and roof lantern or a pitched tiled roof.
How much does a single-storey extension cost in Northamptonshire?
In Northamptonshire, a single-storey extension typically costs £1,850–£2,750 per m² of floor area, including VAT at 20% (standard rate). For a typical 20 m² project that is roughly £37,000–£55,000. The guide for Northamptonshire reflects typical costs in the East Midlands, roughly 5% below the UK-wide figure. Design, structural calculations and council fees are extra.
| Size or type | Guide range |
|---|---|
| Smaller project (14 m²) | £26,000–£38,500 |
| Typical project (20 m²) | £37,000–£55,000 |
| Larger project (28 m²) | £52,000–£77,000 |
| Rate per m² | £1,850–£2,750 |
The biggest cost drivers are:
- Ground conditions and foundation depth
- Floor area and the length of new wall
- Roof type and the size of any lantern or roof windows
- How much of the existing house is opened up
Your exact price depends on the survey and drawings. The guide price tool gives a quick range in the meantime.
Do you need planning permission for a single-storey extension in Northamptonshire?
Here is how the rules work for a single-storey extension in England. In England a single-storey rear extension is often permitted development: broadly up to 3 metres deep on a semi-detached or terraced house and 4 metres on a detached house, no higher than 4 metres, with eaves no higher than 3 metres within 2 metres of a boundary, and covering no more than half the garden. The larger home extension scheme allows up to 6 or 8 metres if you notify the council first and neighbours have a chance to comment. In conservation areas, National Parks and National Landscapes the larger home extension route, side extensions and cladding are not permitted development. Flats and houses whose rights have been removed need a planning application, and listed buildings need listed building consent.
Where work involves a shared wall, building on the boundary, or digging foundations within 3 metres of a neighbour’s building and deeper than its foundations (or within 6 metres for deeper work), the Party Wall etc. Act 1996 applies and notices must be served before work starts.
At the survey we check your house’s planning history and any restrictions, and tell you which route applies.
What we handle
On a typical single-storey extension we handle:
- Doors, windows and roof glazing installed
- Knock-through to the existing house with steel beams
- Cavity walls, insulation and roof to current building regulations
- Measured survey and design drawings
- Structural calculations for steels and lintels
- Foundations, drainage and any build-over agreement
How long does it take?
Build time is typically 10 to 14 weeks on site for a typical rear extension, after approvals. Allow time beforehand for drawings and approvals, usually 6 to 12 weeks.
Building in Northamptonshire
The East Midlands has a mix of red-brick terraces, inter-war semis and newer estates, so rear extensions and garage conversions are common projects.
Towns we cover in Northamptonshire include Northampton, Corby, Kettering, Wellingborough, Rushden and Daventry.
How it works
- 01
Free visit
We look at the property, talk through what you want and measure up. There is no charge and no obligation.
- 02
Drawings and written quote
We prepare the design and give you a written, itemised quote with inclusions and exclusions.
- 03
Permissions sorted
Applications, building control and neighbour arrangements, all before the start date.
- 04
Build and handover
Work follows the agreed programme, with inspections along the way and a completion certificate at the end.
Questions about single-storey extensions in Northamptonshire
How long does a single-storey extension take?
Once approvals are in place, a typical rear extension takes 10 to 14 weeks on site. Design and approvals usually take another 6 to 12 weeks beforehand, depending on whether a planning application is needed.
Do I need a party wall agreement?
If you are in England or Wales and the work involves a shared wall, building on the boundary or digging foundations close to a neighbour, the Party Wall etc. Act 1996 applies and notices must be served before work starts. We advise on this at the design stage.
Do you build single-storey extensions in Northamptonshire?
Yes. We build single-storey extensions across Northamptonshire, including postcodes CV23, LE15, LE16, MK19 and NN1. Every project starts with a free home visit.
How much does a single-storey extension cost in Northamptonshire?
In Northamptonshire, a single-storey extension typically costs £1,850–£2,750 per m² of floor area, including VAT at 20% (standard rate). A typical 20 m² project comes to roughly £37,000–£55,000. Your fixed price depends on the survey and drawings.
What is a build-over agreement?
If a public sewer runs within 3 metres of the extension, the water company normally has to agree to the work before it starts. We check sewer records during design and apply where needed.